1. Field of the Invention
This invention relates to lock-up control devices that control lock-up clutches provided in power transmission systems of cars equipped with automatic gears. This application is based on Patent Application No. Hei 9-269060 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
The power transmission system of the car uses both of a torque converter and a lock-up clutch so as to improve power transmission efficiency. The conventional lock-up control device, provided for the power transmission system of the car, is designed to use torque amplification effect of the torque converter (or deceleration effect) by adjusting engagement force of the lock-up clutch in response to situation in revolutions of the engine. In addition, the paper of Japanese Patent Application, Publication No. Hei 9-32915 discloses an example of the lock-up control device, which is designed as follows:
Based on depression of an accelerator pedal, a decision is made as to whether a driver has an intention to accelerate the car or not. If the driver has such an intention to accelerate the car, the lock-up control device releases an engaged state of the lock-up clutch, so that the power transmission system is placed in a power transmission state using the torque converter. Thus, the lock-up control device is capable of using torque amplification effect of the torque converter for acceleration of the car.
The above lock-up control device uses the torque amplification effect of the torque converter according to needs to improve acceleration performance of the car. In addition, the lock-up control device is capable of further improving the acceleration performance by adjusting deceleration ratio (or torque amplification ratio) of the torque converter based on variation ratio of engine load such that the engine can demonstrate the maximum driving force.
In general, it is preferable to obtain good fuel efficiency by employing an efficient way for transmission of the driving force of the engine. In order to do so, it is necessary to maintain the lock-up clutch as tightly as possible, in other words, it is necessary to place the lock-up clutch in an engaged state, so that utilization of the torque amplification effect of the torque converter is made minimal, wherein the torque amplification effect is utilized by releasing the engaging of the lock-up clutch.
In some case, a driver demands the car to make a high degree of acceleration corresponding to a high level of torque, which exceeds an amount of torque that is obtained by using the aforementioned torque amplification effect of the torque converter. In order to obtain such a high level of torque that is demanded by the driver, it is necessary to perform shift-down operation. Herein, acceleration performance of the car is greatly influenced by a manner to perform the shift-down operation. In general, the shift-down operation is performed in response to two stages of decisions, which are adequately made in different procedures. According to a first procedure, the device makes a decision such that the shift-down operation is performed in accordance with a shift map, which shows optimum shift positions on the basis of the relationship between the throttle opening and car velocity. According to a second procedure, a difference is detected between target torque and maximum torque corresponding to the above shift position, so if the engine does not have a room to increase engine torque thereof, the device makes a decision to perform the shift-down operation. Incidentally, the aforementioned two stages of decisions will be referred to as a first decision using the shift map and a second decision based on excessive engine torque, for example. By the way, gear-change boundary lines are drawn on the shift map to indicate events that gear-shift operations are performed. In other words, the shift-down operation is not performed unless the present point on the map representing the present relationship between the throttle opening and car velocity crosses a certain gear-change boundary line. For this reason, if the aforementioned first decision using the shift map is given a great weight, the shift-down operation does not follow up with accelerator pedal operation. As a result, drivability will be deteriorated. In contrast, if the second decision based on the excessive engine torque is given a great weight, the shift-down operation should be performed even in the case of the instantaneous shortage of the engine torque. In addition, the excessive engine torque occurs due to the shift-down operation, so shift-up operation is performed. Thus, the shift-up and shift-down operations are repeated, which is called a shift busy phenomenon.